Description of the Invention Background
As is known in the art, powered eccentric orbit sanding and grinding devices provide a means for sanding and grinding the surfaces of workpieces of various types of material. Sanding and grinding, as well as other similar operations wherein material is removed from a surface of a workpiece are collectively referred to herein as "abrading". Also, as used herein, an "eccentric orbit" sanding or grinding device is one having an abrading surface for removing material from a workpiece and the abrading surface both rotates and orbits simultaneously. More specifically, the abrading surface rotates about a rotational axis, and the rotational axis simultaneously orbits about a point offset from the rotational axis. As a point of reference only, the combined rotational and orbital motions of the abrading surface of an eccentric orbit abrading device may be compared generally with the motion of the Earth, which rotates about its axis while also orbiting about the Sun.
The combination of rotational and orbital motions of the abrading surface of an eccentric orbit abrading device is advantageous for at least the reason that during use a point on the abrading surface is less likely to describe a repeat pattern relative to the abraded surface of the workpiece than, for example, a point on the belt of a belt sander or the abrading disk of a disk sander in which the disk rotates about an axis which does not orbit about an offset point. If the abrading surface of an abrading device contacts the workpiece and a point on the abrading surface describes a repeat pattern on the workpiece, sand grains or other particles of abrasive material resident on the abrading surface may leave grooves, indentations, or other unsightly lines on the workpiece and mar its surface finish. Familiar examples of abrading devices having abrading surfaces that have a high tendency to generate repeat patterns include rotational disk sanders (that is, rotating sanders that do not provide for orbital motion of the rotational axis of the abrading surface) and powered belt sanders. The combined rotational and orbital motions of the abrading surface of an eccentric orbit abrading device may reduce the occurrence of such unsightly lines because the more complex motion described by a point on the abrading surface lessens the likelihood that repeated patterns will occur.
A subset of eccentric orbit abrading devices are the random orbit abrading devices. The abrading surface of random orbit abrading devices also moves in a combined rotational/orbital motion as described above, but the abrading surface freely rotates about the above-described rotational axis and is not positively driven to rotate. If left unchecked, the impulse imparted to the abrading surface as it orbits around the point offset from the surface's rotational axis causes the abrading surface to rotate about the rotational axis, and the rate of rotation of the abrading surface about the rotational axis may match the rate at which the abrading surface orbits about the axis point offset from the rotational axis. Reaching such a rotational rate is facilitated by, for example, mounting the abrading pad on a shaft that is received by low friction bearings. The shaft then defines the rotational axis and a means is provided for imparting the orbital motion to the surface. As the abrading surface contacts the workpiece with varying pressures, the frictional forces generated against the surface's rotation will vary the rotational speed of the pad and will prevent it from approaching the surface's orbital speed. The abrading surface's combined varying rotational speed and relatively constant orbital speed results in the random movement of points on the abrading surface, and this feature of random orbit abrading devices further reduces the possibility of sanding lines or other indentations being generated during the abrading operation. When sufficiently random, the action of a powered abrading device may simulate hand sanding, but will remove material from the workpiece at a substantially greater rate and will significantly speed the abrading operation.
A variety of hand -held eccentric and random orbit abrading devices are known in the art. Such known devices typically incorporate an arrangement of two disc members driven by a motor shaft. The motor shaft is coupled to a small electric drive motor, thus providing for hand-held operation of the device. Typically, a first disc member is coupled to the motor shaft and rotates with the motor shaft. The motor shaft thereby defines a rotational axis for the first disc member. A second disc member is rotatably mounted on the first disc member, typically received in low friction bearings, so as to substantially freely rotate relative to the first disc member. The second disc member's rotational axis is offset from the rotational axis of the first disc member. By this arrangement of elements, the second disc member may both rotate about its own rotational axis and revolve or "orbit" about the rotational axis defined by the motor shaft, thereby providing the second disc member, to which abrasive material is mounted, with the aforementioned combined rotational and orbital motions. As noted, if the second disc member may freely rotate about its rotational axis, the movement of points on the abrading surface will be random in use, and such random movement will significantly reduce the likelihood of scarring or gouging of the workpiece caused by the generation of repeat patterns.
The known hand-held eccentric and random orbit abrading devices are subject to several inherent shortcomings. For example, when working with relatively small wooden workpieces, the operator of hand-held eccentric or random orbit sanding devices typically may operate the device with one hand while manipulating and adjusting the orientation of the workpiece with the other hand. In such circumstances, it may be difficult to steady the workpiece, and safety concerns also arise because of the risk that the operator's hand may contact the driven sanding surface, resulting in possible injury. Additionally, the known handheld eccentric or random orbit sanding devices provide little or no precision in the control of the angle at which the workpiece contacts the device's sanding surface. Such a drawback is particularly troublesome when sanding small or easily abraded workpieces or when sanding adjacent workpiece surfaces that meet at an angle which the operator wishes to maintain in the finished article.
To address the foregoing shortcomings of the known hand-held eccentric and random orbit abrading devices, additional equipment such as a jig or vise have been used to immobilize the workpiece in a desired orientation and allow the operator to use both hands to manipulate the abrading device. Certain jigs or vises also have been employed to ensure that the abrading surface of hand-held abrading devices contact the workpieces at specific angles. The jigs and vises require additional expense, require time for proper mounting of the workpiece, exert pressure on the workpiece that may mar its surface, and significantly complicate the abrading process.
A further drawback of known hand-held eccentric and random orbit abrading devices is that as they approach a particular size and/or weight, they become difficult or impossible to use. An operator cannot readily manipulate large and/or heavy hand-held devices, and the vibrations and inertial forces generated by the combined rotational and orbital movements of the abrading surface makes the steady handling and accurate positioning of the device relative to the workpiece increasingly difficult. Certain known hand-held eccentric and random orbit abrading devices incorporate counterbalance weight means that will to some extent offset the vibrational forces generated by the eccentric rotation of the abrading surface about the motor shaft. However, as the size of the abrading surface of such devices becomes greater, the vibrational forces generated as the abrading surface eccentrically rotates about the motor shaft become increasingly significant. Dampening of such forces by counterbalance weight means eventually becomes impractical because the weight means significantly augments the weight of the device.
Accordingly, considering the deficiencies of the known hand-held eccentric orbit abrading devices, the need exists for an improved eccentric orbit abrading apparatus.